Battery electrodes (plates) used in lead-acid batteries are made by applying a paste made of a lead compound to the surface of a battery plate and electrochemically forming the paste into an active material. Such pastes typically contain lead, lead oxide(s), basic lead sulfate compounds, and water.
In general, the paste is made by adding sulfuric acid and water to a mixture of lead and lead oxide(s) to form basic lead sulfate compounds in a mixture with excess unreacted lead oxide and lead. According to one known process, this is done by first weighing out a predetermined amount of lead oxide into a weigh hopper and dumping the lead oxide into a batch mixer, such as a mulling mixer. Dry additives such as fiber and expander are directly added into the mixer. The resulting mixture is dry mixed for several minutes so that the fiber and expander are dispersed throughout the oxide. Water is then added as needed to make a paste of the desired consistency. Excessively moist or dry paste render pasting impossible. The wet mixture is mixed for a short time to wet out the lead oxide. Sulfuric acid is then added as mixing continues until the temperature peaks at about 65.degree. C. and then drops to the range of 43.degree.-49.degree. C. The acid is added gradually to prevent the paste from overheating. The resulting paste is then cooled by evaporation of water and conduction to the mass of the mixer. Such a lead-acid battery paste is generally made in a batch reactor, although continuous processes have been suggested.
Many variations of this process have been proposed. In one method, a pre-sulfated paste material containing basic lead sulfate, e.g., tri- and tetrabasic lead sulfates (3PbO.PbSO.sub.4.H.sub.2 O and 4PbO.PbSO.sub.4), is made in dry form prior to forming the paste. See Malloy U.S. Pat. No. 3,194,685, issued July 13, 1965, Johnstone U.S. Pat. No. 2,182,479, issued Dec. 5, 1939, and Weir U.S. Pat. No. 1,572,586, issued Feb. 9, 1926. Monobasic lead sulfate has also been used as a presulfated paste material. Large crystals of monobasic lead sulfate are formed by sulfurizing an aqueous solution of basic lead acetate with, for example, amido sulfonic acid. The monobasic lead sulfate product is then dried prior to its use in preparing a paste mix. See, for example, Voss et al. U.S. Pat. No. 3,169,890, issued Feb. 16, 1965.
Lead oxide has been reacted with ozone to form improved lead oxides useful as active materials in batteries, as described in Parker U.S. Pat. No. 4,388,210, issued June 14, 1983 and Mahato et al., U.S. Pat. No. 4,656,706, issued Apr. 14, 1987. Persulphate treatments have also been used to convert lead oxide to lead dioxide in battery plates. See Reid U.S. Pat. No. 2,159,226, issued May 23, 1939.
In another process a reactor for continuously producing a sulfated lead oxide includes a continuously operating mixer into which a pulverized filler (lead-lead oxide powder) is fed by a pneumatic duct or screw conveyor. Sulfuric acid is sprayed into the dry mixture to form the sulfated reaction product. Water is later added to form the battery paste. See, e.g., Jache U.S. Pat. No. 3,449,166, issued June 10, 1969.
Biagetti U.S. Pat. No. 3,765,943 emphasizes the advantages of preparing a tetrabasic lead sulfate from orthorhombic lead oxide. The lead oxide starting material and an expander are mixed with aqueous sulfuric acid so that the reaction is carried out in aqueous suspension. See also Biagetti et al., Bell System Technical Journal, September, 1970, No. 49, pp. 1305-1319, wherein the pastes are pre-diluted with water just prior to application to cell grids. Positive plates prepared according to such a procedure generally exhibit good performance and cycle life. However, positive plates prepared from such presulfated paste mixes are difficult to form; see, for example, Yarnell and Weeks, J. Electrochem. Soc., No. 126, p.7 (1979). Such plates must usually be cured for at least 24 hours before being formed.
Prior art batch processes suffer from various disadvantages. The mixing vessel must be kept clean to avoid jamming due to dried paste left over from a previous batch. Cleaning is also needed in order to switch from a negative plate batch to a positive plate batch because the negative plate additives reduce the performance of positive plates. Batch methods also generally require a dry mixing step prior to the sulfate-forming reaction step. The present invention addresses these disadvantages.
One aspect of the present invention utilizes a dewatering apparatus in a process for making battery paste. Many such devices, such as belt presses, are known and have been used to dewater compositions such as sludges. Davis et al. U.S. Pat. No. 4,697,511, issued Oct. 6, 1987, Emson et al. U.S. Pat. No. 3,974,026, issued Aug. 10, 1976, Davis U.S. Pat. No. 4,475,453, issued Oct. 9, 1984, Dahl U.S. Pat. No. 4,705,602, issued Nov. 10, 1987, Wohlfarter U.S. Pat. No. 3,942,433, issued Mar. 9, 1976, Bastgen U.S. Pat. No. 4,019,431, issued Apr. 26, 1977, and Hakansson et al. U.S. Pat. No. 4,501,669, issued Feb. 26, 1985 exemplify such devices. The present invention advantageously employs a dewatering device in an apparatus for making battery paste.